Broadly speaking excavation cranes or excavators can be separated into three basic classes: a trencher; a shovel; and a skimmer. Each includes a segmented boom. The boom is constructed either with a single pivot support for a swinging shovel (as in a trencher or shovel) or in the form of two rails set close together on which runs a wheeled carriage with a fixed shovel or skimmer.
In the skimmer a fixed shovel on a wheeled carriage is used to skim a load of soil off the ground level such as piled material for loading into trucks. When working as a digging shovel or excavator, the running carriage is replaced by an arm, pivoted to some point midway along the boom, and carrying the shovel at its lower end. The excavator is driven, with the boom raised, to the earth bank on which it is required to operate, until the shovel hanging at the end of its arm, is in a position to scoop up its load. The operating mechanism pulls the shovel up forwards and upwards in an arc. Steel cutting teeth set on the edge of a shovel then cut their way into the bank until the shovel is filled. The shovel is then swung over the loading truck and emptied as the case may be.
The trencher works on a similar principle, but in this case the shovel arm is pivoted at a point about one-third from one end, to the top of the boom with the shovel opening and the cutting teeth facing the operator. The trencher excavator is driven into position at the end of the trench. By means of the boom operating mechanism, it is pulled so that it swings the shovel away from the operator. The boom is then lowered until the shovel cutting teeth are in correct position; then it is pulled towards the operator causing it to describe an arc at the end of its arm and to dig downwards into the ground.
The principal point being in each of these cases is that the cutting teeth or digging edge of the bucket or scoop is driven in a fixed circle. Consequently, the cutting teeth or digging edge can be angled to an optimum position to maximize the soil fracturing effect thereby optimizing the digging effect of the shovel or scoop.
Excavators are not restricted to one axis of rotation. That is, the bucket or scoop may be rotated about more than one horizontal axis at the same time. In particular, most excavator buckets or scoops usually have at least two, if not three, horizontal axes of rotation. Consequently, the leading edge or digging edge of the bucket or scoop cannot be made to bite into the ground to be excavated at the optimum angle of attack. One expedient to resolve this problem is to incorporate soil penetrating devices on the leading edge of the bucket such that the angle of attack is broadened thereby reducing the reaction forces applied to the bucket or scoop as it excavates soil.
In addition, the orientation of the bucket or scoop has a significant effect on the force needed to remove or break up the soil.
It is common experience to observe a trencher or shovel excavating ground and see the forward portion or rearward portion of the machine base lift off the ground. This is in response to the reaction forces of the soil being applied to the bucket or scoop and along the boom to the base of the crane. This is one indication that the leading edge or the digging edge of the bucket or scoop is not aligned in its optimal direction relative to the soil to be removed.
Preferably, the orientation of the leading edge of the scoop or bucket should be parallel to and coincident with the line of maximum net force exerted by the scoop on the embankment being penetrated. Such a design would also result in evenly distributed wear on the surfaces of the excavation scoop. Furthermore, it would tend to eliminate the bending stress at the leading edge or lip of the scoop when maximum digging forces are imposed thereon. These facts were recognized by Thompson (U.S. Pat. No. 3,896,569); however, it was believed that proper tooth orientation could be factored into design and that fixed, non-pivoting teeth were satisfactory. The problem experienced in using excavators whose buckets may be pivoted about two or more parallel axis of rotation was not addressed.
It is not uncommon that excavation is complicated by poor soil conditions. Often the ground to be worked is frozen, rocky or extremely well packed or dense. In such instances, some means must be provided to break up the hardened earthen material before excavation. Rasmussen (U.S. Pat. No. 1,919,075) described a grubbing tool for tree removal prior to soil cultivation. Although movable teeth are used no suggestion is made that the teeth serve any purpose other than entrainment of roots and other tangled debris. It operated much like a "rake."